1. Field of the Invention
The present invention generally relates to a multilayer structure for reflecting x-ray radiation and an associated method for analyzing the atomic or molecular composition of a sample through x-ray fluorescence spectroscopy.
2. Description of Related Art
Multilayer reflectors, or Bragg reflectors, operating with x-ray radiation, are often utilized for analyzing structures and detecting the absence or presence of particular atomic elements or molecules. One such procedure is generally known as x-ray fluorescence spectroscopy (XRF). Such a procedure is useful in detecting impurities of minimal amounts present in the sample of interest. For example, XRF is used in the semiconductor industry for detecting impurities in the silicon and germanium wafers that are the foundation of highly-integrated circuits. Two types of XRF systems are commonly used. In energy dispersive spectroscopy (EDS), a detector receives a broad range of energy from the sample and the detector asked to discriminate based on the wavelengths of interest. In wavelength dispersive spectroscopy (WDS), a collection optic acts as a filter to relay to the detector only a wavelength of interest. Each approach has its advantages and disadvantages. For example, EDS systems can detect wavelengths over a large range but have sensitivity limitations. WDS systems have high energy resolution and sensitivity but are designed for detecting only wavelengths of specific elements of interest.
In a typical XRF/WDS procedure, an x-ray radiation field is guided to a sample, such as a silicon wafer. The impinging radiation induces a fluorescent radiation field, which is incident upon a multilayer or Bragg reflector. The fluorescent radiation field is directed by the multilayer to a measuring or analyzing detector.
In a WDS system, the multilayer functions both as a reflective optic and a frequency selector because the multilayer is designed and oriented in a system such that fluorescent radiation that satisfies Bragg's equation is reflected. Bragg's equation in general is:nλ=2d sin θ,  (1)
where n is an integral number, λ is the wavelength of the initial x-ray radiation field, d is the periodicity of the lattice structure of the multilayer, and 2θ is the angle of diffraction.
Bragg's equation is satisfied for certain types of natural crystals that have regular lattice structures. However, typical crystals have spacings of a few tenths of a nanometer, and because soft x-rays have wavelengths between 1-10 nanometers, Equation (1) is not satisfied for such wavelengths. Consequently, for soft x-ray analyses using Bragg-type reflections, a multilayer or “synthetic crystal” reflector is necessary.
A typical multilayer consists of a substrate upon which layers of two or more different materials are sequentially deposited, forming a period of layers of thickness d. Generally, one of the materials has a high dielectric constant and the other has a low dielectric constant. Upon impinging at that interface between the dielectric constants, approximately 10−2 to 10−3 of the incident radiation is reflected at each period of the layers. Therefore, a multilayer structure having 10 to 103 layers would theoretically reflect nearly all of the incident radiation. Multilayers have the added advantage of customization, meaning that the d-spacing can be tailored to meet Bragg's equation for different wavelengths of interest.
Traditionally, multilayer XRF analyzers have been utilized in the analysis of various elements from magnesium (Mg) to beryllium (Be). For example, in the semiconductor industry, semiconductor material substrates are now being fabricated with multiple thin film layers. Multilayers can be used to characterize samples having multiple thin film layers formed thereon. Each film layer can be formed from a different material. A wide variety of material combinations having a d-spacing ranging from 1.5 nm to 10 nm are currently in use. For the analysis of a particular element, one can find an optimal structure for the best available performance, but there is always a demand for improvements. For example, a common multilayer consisting of tungsten-silicide (W/Si) periods has been used to analyze films containing various elements such as magnesium (Mg), sodium (Na), fluorine (F), and oxygen (O). Such analyzers are relatively efficient in the analysis of magnesium (Mg) and sodium (Na), but their performance in analyzing fluorine (F) and oxygen (O) is less efficient. By developing a deposition technology one can minimize imperfections of the W/Si multilayer structures such as interlayer diffusion, roughness and others, but a gain in performance is expected to be minor because of a fundamental limit arising from the optical constants of tungsten (W) and silicon (Si) materials. Therefore, there is a need for an improved multilayer analyzer for analyzing thin film layers containing various elements, and, in particular, for analyzing elements such as fluorine (F) and oxygen (O).
Examples of thin film layer materials commonly fabricated on semiconductor substrates further include oxides, nitrides, titanium (Ti) and titanium-nitride (TiN). Current analysis of TiN thin films, for example, uses two analyzers: a multilayer, such as a scandium (Sc)-based multilayer, for analyzing nitrogen (N); and a crystal, such as lithium fluoride (LiF), for analyzing titanium (Ti). Using a Sc-based multilayer for analyzing N results in significant reflectivity. This latent reflectivity increases the background signal in certain silicon-containing samples, such as silicon wafers. Further, the Sc-based multilayer is only capable of analyzing the N in TiN thin film coated samples, thus requiring the second analyzer (LiF crystal) for analyzing the Ti. Even when N is the only analyzed element, both analyzers have to be used due to the presence of the Ti-LI, line, which is in close vicinity to the N—Kα line. This requires a two-channel device for housing two separate analyzers which is expensive and inconvenient. Therefore, there is a need for a single analyzer for WDS for analyzing samples fabricated with TiN thin films.
Currently, pure magnesium (Mg)-based structures, such as silicon carbide/magnesium (SiC/Mg) multilayer structures, are used for applications at energies below 50 eV. With such low energies, these structures have d-spacings larger than 15 nm. However, for XRF analysis of light elements from magnesium (Mg) to nitrogen (N), multilayer analyzers with d-spacings of between about 2 and 4 nm are optimal. Multilayer structures containing pure Mg layers, such as tungsten/magnesium (W/Mg) with a d-spacing of about 4 nm and smaller, are not desirable due to the strong intermixing/reaction of these materials which results in a very poor x-ray performance.
Thus, there is a need for improving the quality of existing analyzers, or for lowering the number of analyzers in a WDS spectrometer without narrowing the number of analyzed elements and losing too much in performance.
Further, there is a need for an improved multilayer analyzer which has the capability to optimize parameters, such as reflectivity and selectivity, as well as reduce undesirable background signals, depending upon the particular application and the elements under analysis.